1. Field of the Invention
This invention relates to a positive-chargeable toner used in recording processes such as electrophotography, electrostatic recording, magnetic recording and toner-jet recording, an image forming method having the step of developing an electrostatic latent image by the use of the toner, and an apparatus unit having the toner.
2. Related Background Art
A number of methods as disclosed in U.S. Pat. No. 2,297,691 and Japanese Patent Publication No. 42-23910 and No. 43-24748 are conventionally known as electrophotography. In general, copies are obtained by forming an electrostatic latent image on a photosensitive member by utilizing a photoconductive material and by various means, subsequently developing the latent image by the use of a toner, and transferring the toner image to a transfer medium such as paper as occasion calls, followed by fixing by the action of heat, pressure, heat-and-pressure, or solvent vapor. The toner not transferred and having remained on the photosensitive member is cleaned by various means, and then the above process is repeated.
In recent years, such copying apparatus, reflecting commercial needs for composite type machines and personal use, are severely sought to be made more small-sized, more light-weight, more high-speed and more highly reliable. As a result, a higher performance has become required also for toners.
For example, various methods and devices have been brought out in relation to the step of fixing a toner image to a transfer sheet such as paper. A method most commonly available at present is the heating pressure-fixing system using a heat roller. The heating pressure-fixing system using a heat roller is a method of fixing a toner image by causing the toner image on an image-receiving sheet to pass the surface of a heat roller whose surface is formed of a material having releasability to toner while the former is brought into contact with the latter under application of a pressure. Since in this method the surface of the heat roller comes into contact with the toner image of the image-receiving sheet under application of a pressure, a very good thermal efficiency can be achieved when the toner image is fixed onto the image-receiving sheet, so that the toner image can be fixed rapidly.
The heat-roll fixing having been hitherto widely used, however, is required to maintain the heat roller at an optimum temperature in order to prevent faulty fixing from being caused by the variations of the heat-roller temperature that may occur when the transfer medium is passed or because of other external factors, and also to prevent what is called the offset phenomenon in which the toner moves to the heat roller. This makes it necessary to make large the heat capacity of the heat roller or a heater element, which requires a large electric power and also requires a larger size of image forming apparatus or causes in-machine temperature rise.
Accordingly, for the purpose of causing no toner to adhere to the surface of the fixing roller or improving low-temperature fixing performance, measures have been proposed in variety. For example, the roller surface is formed of a material having an excellent releasability to toner (e.g., silicon rubber or fluorine resin) and, in order to prevent offset and to prevent fatigue of the roller surface, the roller surface is further covered with a thin film formed using a fluid having a good releasability as exemplified by silicone oil. However, this method, though very effective in view of the prevention of the offset of toner, requires a device for feeding an anti-offset fluid, and hence has also the problem that fixing assemblies must be made complicated and apparatus must be made large-sized.
Thus, in addition to such approaches from fixing assemblies, measures very greatly rely on the properties of toner in order to accomplish a fixing method having a good efficiency while achieving a good fixing performance of visible toner images to the transfer medium and good anti-offset properties.
Namely, especially from the viewpoint of anti-offset techniques, it is not a preferable measure to prevent the offset by feeding the anti-offset fluid. Rather, under existing circumstances, it is sought to further provide a toner having a broad low-temperature fixing range and high anti-offset properties. Accordingly, in order to improve release properties of the toner itself, it has been attempted to add a wax such as low-molecular-weight polyethylene or low-molecular-weight polypropylene that may well melt at the time of heating. The use of wax is effective for preventing offset, but on the other hand makes the toner have higher agglomerating properties and also makes charging performance unstable, to tend to cause a lowering of developing performance at the time of running. Accordingly, as other methods, it is also attempted to improve binder resins.
For example, a method is known in which, in order to prevent offset, the glass transition temperature (Tg) and molecular weight of a binder resin in toner are made higher to improve the melt viscoelasticity of the toner. When, however, the anti-offset properties are improved by such a method, although the developing performance is not so affected, an insufficient fixing performance may result to cause the problem of poor fixing performance in low-temperature fixing, i.e., low-temperature fixing performance, which is required for the achievement of high-speed copying and energy saving.
In order to improve the low-temperature fixing performance of toner, it is necessary to make the toner have a low viscosity at the time of its melting and make large the area of contact with a fixing member. For this reason, it is required to make lower the Tg and molecular weight of binder resins used.
That is, the low-temperature fixing performance and the anti-offset properties conflict with each other in some phase, and hence it is very difficult to provide toners satisfying these performances simultaneously.
To solve this problem, for example, a toner comprising a vinyl polymer cross-linked to an appropriate degree by adding a cross-linking agent and a molecular-weight modifier is proposed, as disclosed in Japanese Patent Publication No. 51-23354. Also, toners comprising a blend type resin vinyl polymer in which Tg, molecular weight and gel content are specified in combination are also proposed in a large number.
Such toners incorporated with the cross-linked vinyl polymer or gel content show an excellent effect on the anti-offset properties. However, when such a cross-linked vinyl polymer is used as a toner material to incorporate it in the toner, the polymer may undergo a very great internal friction in the step of melt kneading when the toner is produced, and a great shear force is applied to the polymer. Hence, in most cases, breaking of molecular chains may occur to cause a decrease in melt viscosity to adversely affect the anti-offset properties.
Accordingly, to solve this problem, as disclosed in Japanese Patent Application Laid-open No. 55-90509, No. 57-178249, No. 57-178250 and No. 60-4946, it is disclosed to use as toner materials a resin having a carboxylic acid and a metal compound, which are heated and reacted at the time of melt-kneading to form a cross-linked polymer and incorporate it in the toner.
Japanese Patent Application Laid-open No. 61-110155 and No. 61-110156 disclose that a binder having as essential constituents a vinyl resin monomer and a special monoester compound is allowed to react with a polyvalent metal compound to effect cross-linking through a metal.
Japanese Patent Application Laid-open No. 63-214760, No. 63-217362, No. 63-217363 and No. 63-217364 disclose that a binder resin has a molecular weight distribution separated into two groups, a low-molecular weight region and a high-molecular weight region, and carboxylic acid groups incorporated into the low-molecular weight region side are allowed to react with polyvalent metal ions to effect cross-linking (a dispersion of a metal compound is added in a solution obtained by solution polymerization, followed by heating to carry out the reaction).
Japanese Patent Application Laid-open No. 2-168264, No. 2-235069, No. 5-173363, No. 5-173366 and No. 5-241371 disclose toner binder resin compositions and toners in which the molecular weights, mixing ratio, acid values and percentages of low-molecular weight components and high-molecular weight components in binder resins are controlled to improve fixing performance and anti-offset properties.
Japanese Patent Application Laid-open No. 62-9256 discloses a toner binder resin composition comprising a blend of two kinds of vinyl resins having different molecular weights and acid values of resin.
These proposals set forth in the foregoing can certainly be very effective in respect of the improvement in anti-offset properties, though having merits and demerits. These, however, require introduction of acid value into toner binders, and hence negative chargeability is necessarily imparted to toners, though having more or less differences. As the result, when applied in positive-chargeable toners, their charging performance at the rise of toner charging, during running and in an environment of high humidity or low humidity may be greatly damaged to cause a lowering of developing performance concerning image density and fog. Moreover, they can not stably retain a proper charge quantity to make toner agglomeration properties higher, and have not attained satisfactory results.
Meanwhile, toners must have positive or negative charges in accordance with the charge polarity of electrostatic latent images to be developed, and hence it is commonly known to add dyes, pigments or charge control agents to toners. Among these, as positive charge control agents, it is known to use quaternary ammonium salts or lake pigments of these, polymers having a tertiary amino group or quaternary ammonium salt in the side chain, triphenylmethane dyes and lake pigments of these, Nigrosine, and products modified with fatty acid metals salts.
These conventional positive charge control agents, however, have tended to be not able to impart sufficient charge quantity to toners, or, if they are able to impart sufficient charge quantity, tended to be affected by other constituent materials of toner to cause occurrence of blotches due to excessive triboelectricity of toner or non-uniform charging, or make toners have higher agglomerating properties, or cause deterioration of developing performance, e.g., image density decrease and fog. This tendency is especially remarkable in positive-chargeable toners having an acid value. Moreover, there has been a problem of sleeve contamination which is caused when the charge control agent comes off toner particles to stick to the surface of the developing sleeve, a developer carrying member.
Japanese Patent Publication No. 8-10364 discloses a positive-chargeable toner suited for not only black printing but also color printing, containing 4,4'-methylene-bis(2-alkyl-5-methylimidazole), which is white or pale and also has a high charge control effect by its addition in a small quantity. However, in the toner disclosed in Japanese Patent Publication No. 8-10364, no improvement has been achieved in respect of the improvement in fixing performance of the toner, and the binder resin specifically used in Examples is a styrene-acrylate copolymer. Taking account of the fixing performance of toner, there is room for further improvement.
Japanese Patent Application Laid-open No. 3-71150 discloses that a positive-chargeable toner containing a polyester resin having a softening point of from 70 to 150.degree. C. and an acid value of 5 mg KOH/g or less, synthesized from a diol and a polybasic carboxylic acid, and also containing a specific imidazole derivative has a stable triboelectric charging performance and a sharp and uniform distribution of quantity of triboelectricity, enables development and transfer faithful to latent images, can maintain initial-stage characteristics even when continuously used over a long period of time, may cause no agglomeration of toner and no change in charging performance, and also may not be affected by changes in temperature and humidity to reproduce stable images. However, the toner disclosed in Japanese Patent Application Laid-open No. 3-71150 employs as a binder resin a polyester having relatively a large environmental dependence and negative chargeability, and hence its anti-offset properties may lower when its acid value is made small taking account of the environmental dependence and the charging stability of positive-chargeable toner. Moreover, in this case, since many of the hydroxy groups are left, the toner may largely be affected by changes in humidity in a high humidity. The toner is hardly affected by humidity when used in the two-component developing system, so that serious problem does not occur at all, while the toner is liable to be affected by humidity when used in the one-component developing system. Thus, this toner can not achieve a highly well balanced state of the anti-offset properties, environmental stability and positive charging performance, and there is room for further improvement.
From another aspect, there is a problem of how proper charging can be maintained stably for a long term and in a good efficiency in an instance where a toner is brought into contact with the developing sleeve, the developer carrying member, to triboelectrically charge the toner.
As the developing sleeve in an image forming apparatus employing electrophotography, a member is used which is produced by molding, e.g., a metal or an alloy or compound thereof into a cylinder and treating its surface by electrolysis, blasting or filing so as to have a stated surface roughness. As commonly available substrate materials for the developing sleeve, stainless steel, aluminum and nickel are in wide use, which are disclosed in Japanese Patent Application Laid-open No. 57-66455.
When, however, the positive-chargeable toner is charged using such a developing sleeve, it is difficult to control the charge quantity of toner. For example, when a stainless steel sheet is used as the sleeve substrate material, the developing sleeve has so strong a charge-providing power that the toner present in the vicinity of the sleeve surface may acquire very high charges, so that the toner is strongly attracted to the sleeve surface because of mirror force to undesirably form an immobile layer. This lessens opportunities of friction of the toner with the developing sleeve to inhibit preferable charging. As the result, non-uniform charging of toner or blotches due to excessive charging tend to occur and, of course, developing performance may also deteriorate.
When aluminum is used as the sleeve substrate material, the developing sleeve has a high ability to charge the positive-chargeable toner. However, because of a softness inherent in the material, it has a poor durability and tends to cause image deterioration due to surface wear. Accordingly, in order to endow it with wear resistance, the surface of the aluminum substrate is coated or plated with a metal. Such a technique can improve the hardness of the sleeve surface to make the durability better, but on the other hand most of such sleeves have a low ability to charge the positive-chargeable toner, tending to cause faulty charging of toner.
Similarly, a developing sleeve on the substrate material surface of which is provided with a resin layer has a good durability, but has a limit to the controlling of charge-providing performance to the toner. It has a broad range of application in respect of negative charging, but, when applied in positive charging, can not be endowed with a suitable charge-providing ability. Under existing circumstances, especially when the binder resin has an acid value, it is difficult to charge the toner.